The concept of inhibin as a water-soluble substance produced in the gonad but acting specifically at the pituitary level to inhibit the secretion of follicle-stimulating hormone (FSH) was postulated by McCullagh in 1932 (Science, 76, 19-20). Such preferential regulation of the gonadotropin secretion has generated a great deal of interest and has prompted many laboratories in the past fifty years to attempt to isolate and characterize this substance from extracts of testis, spermatozoa, rete testis fluid, seminal plasma and ovarian follicular fluid, using various bioassays. Although many reports have appeared in the literature claiming the purification of inhibin-like material with molecular weights ranging from 5,000 to 100,000 daltons, subsequent studies have shown that these substances were either not homogenous or did not have the high specific activity expected of true inhibin (de Jong, Molecular & Cellular Endocrin., 13, 1-10 (1979)). Materials having inhibin activity may be used to regulate fertility in mammalian animals, particularly male animals.